Coiled or continuous reel tubing has been used in the oil industry for the last 20-30 years. The fact that it is a continuous single tube provides several advantages when entering a live oil or gas well which could have anything up to 7,000 psi well head pressure. This means the well does not have to be killed, (i.e. a heavy fluid does not have to be pumped down the production tubing to control the oil or gas producing zone by the effect of its greater hydrostatic pressure). Continuous tubing has the advantage of also being able to pass through the tubing through which the oil and/or gas is being produced without disturbing the tubing in place.
Since its introduction, the uses and applications for coiled tubing have grown immensely, and now, rather than just being used to circulate various fluids in a well bore, it is not uncommon for coiled tubing to be used for conveying various hydraulically powered tools and more recently electrically powered tools on its end into the well. This has resulted in conventional electrical wire-line logging cables or small hydraulic conduits being inserted into the inside of the reel of tubing so that these more sophisticated tools can be used and services can be performed.
A disadvantage which has resulted from this practice is the capstan effect of the smaller diameter wire-line or hydraulic tube tending to be pulled very tightly to the inner surface of the continuous reel of tubing. When considering the effect this has on the geometry, it will be appreciated that the wire-line or small hydraulic conductor will have a slightly smaller pitch circle diameter than the larger reeled tubing. The consequence of this is that for each complete 360 degrees the wire-line or hydraulic tube will be slightly shorter in length than the larger reeled tubing, so if this is added up over its total length of 12,000 ft (3657 m) or usually longer, the difference in lengths could be as much as 200 ft (61 m).
This problem has been recognised due to the operational problems encountered. Either one end of the wire-line or hydraulic tube has been pulled out of its connection, or else the reeled tubing itself begins to form a low frequency wave form caused by the tension in the conduit inside the reeled tubing, which prevents the reeled tubing being lowered any deeper into the well without the risk of damaging it.
Another disadvantage with using traditional wire line inside reeled tubing is that it is not compatible with many of the fluids pumped through the reeled tubing, the more common ones being corrosive stimulation fluids, and cement slurries used generally for zonal isolation. The reason for this is that the wire line has two outer reinforcing layers of braided wire followed by an insulation layer protecting the conductors, which typically number up to eight. The normal insulation material is not compatible with the acid systems, although some expensive materials are available, but the total price becomes prohibitively expensive. Alternatively, when pumping cement slurries, the gaps between the braided wire of the cable form natural traps which collect some deposits of the cement slurry, which, when set, either make it difficult for the wire-line to bend. More commonly, the particles of set cement break off leaving residue inside the reel.
Consequently and additionally this has the effect of increasing the weight of the conduit and for conduits having a certain length the conduit is prone to stretching or creep when installed in the well and in particular when it is intended to remain in position for a relatively long period of time for production of the well. The type of conduit for which stretching is a problem depends upon the weight per unit length of the conduit, the material of the conduit and the expected working life of the conduit as well as the nature of the immediate environment surrounding the conduit, although in many applications the invention enables the conduit to be installed in coiled tubing with a dielectric oil between them. Nevertheless stretching of the conduit is frequently a serious problem.